A hybrid of the optical and magnetic information storage systems is the so called magneto-optical information storage system. Such systems can provide increased storage capacity over magnetic systems and allow data to be erased and rewritten, currently a problem in optical systems. It has been estimated that the theoretical upper limit of the storage capacity of magneto-optical systems can be at least as high as 300 megabytes per square inch of media. In practical terms this means that on a single 5.25 inch disk, capacities of approximately 400 to 800 megabytes can be achieved.
In magneto-optical storage, data are recorded and erased on a thin film of magnetic material having properties to be described herein. Similar to magnetic recording, magneto-optical recording stores information in a sequence of magnetic domains whose magnetic polarity is oriented normal to the media surface in either of two possible orientations. The orientations can be thought of as north-pole-up or north-pole-down. An erased disk has all of its magnetic poles pointing in the same direction. The important feature of magneto-optical media is that the magnetic force required to reverse the polarity of the magnetic domains, i.e. the coercive force, varies greatly with temperature. At room temperature, the coercive force necessary to reverse the polarity is so high that an ordinary magnet is too weak. At approximately 150.degree. C., the coercive force required decreases substantially so that information can be recorded using magnetic fields of reasonable strength.
Optical techniques are used in magneto-optical systems to heat selected spots on the media using focused light as the media passes over an electro-magnet, or bias coil, similar to that shown in FIG. 1. In FIG. 1, a portion of a magneto-optical disk 10 is shown to be rotating between an electromagnet 12 and an objective lens 14, forming part of an optical head 15. During a writing operation, the light from a laser source 16 is passed through a polarizing beam splitter 18 and reflected from a mirror 20. The light reflected by the mirror 20 is focused by the lens 14 onto the surface of the disk 10. The heat associated with such focused laser light is sufficient to raise the temperature of the media at the focal point to approximately 150.degree. C.
In this way, a "point" on the media can be heated, lowering the coercive force required to write information at that point. The heated spot within the medium will then take on the polarity of the magnetic flux generated by electromagnet 12. Once the laser beam is turned off, the just heated spot on the media cools, "freezing" the local magnetic orientation of the media due to the resulting increase in the coercive force at that point. To erase information so recorded, the process need only be reversed; that is the point on the media is heated by passing the laser beam through the objective lens 14 so that the magnetic orientation of the media at that point matches the direction of flux generated by the electromagnet.
Reading information so recorded on a magneto-optic disk is achieved by electro-optical means. Again referring to FIG. 1, a lower powered light beam from the laser source 16 is focused onto the media by the objective lens 14. Because of phenomena known as the Kerr magneto-optic effect and the Faraday effect, light reflected from the media (Kerr) or passing through the media (Faraday) will have a slightly different polarization state than that of the incident light focused onto the media. The change in polarization state will be either a clockwise or counterclockwise rotation depending on the magnetic orientation of the media at that point. For further graphical interpretation of the above described magneto-optic operation, reference is made to Freese, Robert P., "Optical disks become erasable", IEEE Spectrum, February, 1988, pages 41-45.
In FIG. 1, light focused onto the disk 10 by the objective lens 14 is reflected back onto the mirror 20 and into the polarizing beam splitter 18. Since the reflected light has a different polarization state than the light generated by the source 16, a portion is reflected by the mirror element contained in the polarizing beam splitter 18 onto the reflective element or interface in the beam splitter 22. Some of the beam is then directed onto a focus and tracking detector 24 for generation of position error signals, described in greater detail below. The remainder of the reflected beam is passed through a beam splitter 26 which is designed to split the light beam into two light beam components which are provided or applied to a pair of photodetectors 28 and 30. The photodetectors 28 and 30 generate electronic signals which characterize data or information stored on disk 10.
Such a differential data detection technique is described in Mansuripur, M., et al., "Signal And Noise In Magneto-Optical Readout", J. App. Phys. 53(6) (June 1982). Typically, data is detected by subtracting the electronic signals from each other in a difference amplifier. The amplifier output is thereafter filtered and processed according to any one of several known techniques. When reading information from or writing information onto a magneto-optical disk, or any optical disk, it is necessary to maintain the radial position of the light beam focused by the objective lens 14 on a particular disk track as the disk 10 rotates. Such an operation is known as track following. Track following requires the generation of a radial position (tracking) error signal. It will also be appreciated from the above that because relatively small magnetic domains will be recorded, read and erased, it is important to maintain a focused spot of light on the desired track. Maintaining the focus of the light beam requires the generation of a focus (axial) error signal. Each of these signals, the tracking (radial position) error signal and the focus (axial position) error signal, have generally heretofore been generated by segmented detectors whose outputs are differenced (subtracted) in various ways to produce these error signals.
Light reflected from a grooved magneto-optical disk which is directed onto the detector 24 will form a sheared interferogram. That is, when light is focused on a spot on a grooved media, such as that used in optical and magneto-optical disks, the reflected light contains a series of orders of diffraction, each having an axis deviated from the central axis. These diffraction orders normally overlap producing the sheared interferogram. When sampled properly, such as with a segmented detector, the sheared interferogram can be used to generate the tracking error signal.
In one previous method of focus detection, the detector in effect senses the diameter of the reflected beam of light, i.e. an illuminated spot, including the sheared interferogram. In other words, focus can be determined by sensing the size of the spot formed by the intersection of the detector with the converging beam. This method is called "spot size detection".
In the generation of such tracking and focus error signals, detector elements in the shape of an "I" have been utilized. Additionally, quadrant type detectors have been proposed for use in determining position error (tracking and focus) signals. See for example, U.S. Pat. Nos. 4,773,053--Gottfried, 4,797,868--Ando and 4,779,250--Kogure, et al.; and Lee, Wai-Hon, "Optical Technology For Compact Disk Pickups", Lasers and Optronics, pp. 85-87 (September 1987).
The problem with such prior techniques for generating tracking error and focus error signals, is the relatively low sensitivity to the state of collimation of the light incident on the detector and high sensitivity to small motions of the reflected beam in relation to the detector. Also such prior systems are not capable of rejecting spatial variations in irradiance in the incoming beam resulting from dirty optical surfaces, lateral motion of the beam or as a result of diffraction of light by optical surfaces upstream from the detector, hereinafter collectively referred to as "pattern noise".
The U.S. Pat. No. 4,862,442, to Tadokoro et al, in reference to FIGS. 6 and 7 therein, discusses a ray tracing diagram based upon the optics organization of the principal parts of a prior art optical play back and recording system, referencing a Japanese Patent Application Laid-open No. 7246/1981.
In Tadokoro et al, light reflected from the surface of an information carrying medium is directed back through an objective lens adjacent the medium into a detector prism having a reflecting surface. Light reflected from that surface impinges upon a photo detector.
The reflecting surface of the detector prism is set so that when the objective lens is correctly focused with respect to the information carrying medium, the angle between the reflecting surface of the detector prism and the incident beam, defined by Tadokoro et al as a pencil of parallel rays, is equal to or slightly lens than the critical angle. If the information carrying medium deviates from the point of focus, the angle of incidence of the light beam incident on the reflecting surface of the detector prism varies around the critical angle. FIG. 7 is referenced to illustrate the extreme sensitivity of the reflecting surface of the detector prism to slight changes in the angle of incidence of the incident light beam near the critical angle.
The effect of departures from focus of the medium, in directions toward and away from the objective lens, upon light reflected and transmitted at the reflecting surface of the detector prism, is discussed at length by Tadokoro et al, noting, particularly, the resulting dead band in which no changes in reflected light beam intensity occur, if the critical angle is exceeded. Light transmitted through the reflecting surface of the detector prism is also discussed, noting the possibility of relocating the photodetector to intercept the transmitted light instead of the reflected light. This approach is negated, however, by Tadokoro et al, observing that in addition to having all of the problems of the arrangement using reflected light, there is the additional problem of an extremely small diameter beam which increases the difficulty of adjustment of the photodetector upon which the transmitted light beam impinges.
Tadokoro et al describe their solution to the problems discussed above in the use of a detector prism which has a multilayer reflecting surface which is stated to have a reflectivity which varies continually with respect to the angle of incidence. This detector prism, having variable reflectivity or transmissivity, is utilized in differing focus error detecting apparatuses, individually using reflected light, transmitted light and both transmitted and reflected light. Although Tadokoro et al describe a focus detection apparatus which uses a transmitted beam, their apparatus improves prior art only in the respect of eliminating "dead band" (discontinuity in the error curve).
Consequently, a need still exists for an apparatus, employing a detector prism to produce complementary light beams by reflections/transmission, for achieving high sensitivity in the production of focus and tracking signals, i.e. position error signals, for reducing sensitivity to small motions of the incident light beam relative to the detector and for rejecting spatial variations of beam irradiance (pattern noise).